Proliferating data and communication signal transmission needs have stimulated the development of multiconductor cables of both extruded and/or laminated constructions. Their application, although as diverse as data bus intra-computer cabinet wiring, peripheral interconnects and under carpet voice and data wiring, share in common problems of cross-talk, signal distortion, attenuation, termination, electromagnetic compatibility and minimum physical size. To meet these parameters, multiconductor cables have been produced by controlling impedance uniformity and encasing them within suitable shields.
One predominant problem is the control of cross-talk within a cable. Cross-talk refers to a phenomenom in which signals are transmitted from an active line to a passive line via electrostatic and electromagnetic fields. Cross-talk and, more particularly, near-end cross-talk is the result of adjacent fields interacting with each other which is usually measured by providing one active pair of conductors or wires which carries a signal, and a passive pair of conductors positioned adjacent the active pair of conductors, with the degree of cross-talk being measured as the degree of signal pickup on the passive pair. Typically, the cross-talk signal level on the passive pair ranges from 10 dB to 100 dB lower than the signal in the active pair. It is this signal pickup which is the result of electrostatic and/or inductive coupling. It will be appreciated that in this patent the terms "wires" and "conductors" are used interchangeably.
Three techniques are commonly used to achieve cross-talk control. The first is referred to as a "ground plane" construction in which a solid or perforated ground plane is placed in intimate proximity to signal conductors to decouple them. The second approach relies on an interposed conductor, referred to as a ground-signal-ground (GSG) system or its paired variant, the ground-signal-signal-ground (GSSG) system to drain off interfering fields. Still a third technique relies on geometric decoupling to preserve the integrity of signal transmission. However, none of these techniques have adequately solved the problem of cross-talk without undesirable tradeoffs.
In the case of GSG construction, the tradeoff is excessive physical bulk in which only one third of the conductors are carrying intelligence, with the other two thirds being ground returns. This results in an excessively wide cable and the need for expensive high conductor count cables.
Ground plane cables are only partially effective in controlling cross-talk, and generally lead to low impedance constructions that can be difficult to drive. Their major other drawback is that they frequently require stripping of the ground plane to terminate the conductors which can lead to an impedance discontinuity at its terminating connection. When the ground plane element itself is cut or abraded the distance varies between it and the conductors from which it is insulated, causing the impedance discontinuity. More importantly with respect to conventional ground plane cables, the grounding or shielding element is planar and therefore does not provide circumferential protection about the individual conductors. Thus, while some signal decoupling is accomplished by the conventional ground plane configuration, substantial leakage occurs between conductors which are adjacent one another such that substantial cross-talk persists.
Geometrically decoupled cables such as illustrated in U.S. Pat. No. 4,381,420 can achieve low cross-talk. However, these cables are extremely dependent on critical spatial relationships of their multiconductors, and as a result are difficult to manufacture within critical tolerances. Part of the problem associated with the process described in this patent is that, while pinch rolling is used, only one of the pinch rollers has a resilient surface, with one or both having preformed grooves therein. The tolerance problem aside, conductors are overlapped on opposite sides of a dielectric, making termination a labor intensive operation. Moreover, in the above-mentioned patent, there is no shielding provided to eliminate cross-talk.
By way of further background, the general nature of a ground plane conductor is an array of individual conductors in one plane, some dielectric medium interposed between the array and the ground plane material itself. The general theory associated with cross-talk reduction by the utilization of a conventional ground plane is that the immediate field surrounding the active conductors will be shunted to the ground plane and to ground, thereby diminishing its effect on adjacent conductors as a result of a capactive coupling. However, in between the adjacent conductors there is no electrostatic shield such that the signals which have only partially coupled to the ground plane are also coupled between adjacent conductors which results in cross-talk. While the conventional ground plane cable assists in draining off some of the spurious electrostatic field, a major part of it is nonetheless present.
U.S. Pat. Nos. 3,312,870; 3,757,029; 3,703,604; 3,818,117; 4,045,750; and 4,340,771 illustrate various ground plane or slotted ground plane configurations. In each case the ground plane in no way is interposed between adjacent conductors.
U.S. Pat. Nos. 3,763,306; 4,143,236; and 4,218,581 illustrate the ground-signal-ground or ground-signal-signal-ground type cross-talk prevention systems. It should be noted that with respect to the last mentioned patent, this is a typical ground-signal-ground configuration in which the conductors are always to one side of the ground plane. Moreover, it will be appreciated that there is no circumferential shield with respect to the signal conductors, with the only member between signal conductors being another conductor which is in contact with the ground plane. The last mentioned conductor is not conformal in that it does not wrap around the signal conductor but simply takes up a little bit of space and provides a small amount of shielding effectiveness.
With respect to the formation of flat cable and referring again to the above-mentioned U.S. Pat. No. 4,381,420, it is very important to note that in order for registration to take place in the above-mentioned patent, the pinch rollers must have grooves in them in order to provide for the conformal nature of the insulating material and the precise conductor spacing. It should be noted that in this patent there is no internal shield whatsoever. The registration of the top and bottom conductors in this patent is extremely critical and must be on the order of .+-.0.002 inches in order for geometric cross-talk control to be effective. Such pitch control is rarely if ever achieved in production with the grooved rollers specified in this patent. The result is that tremendous tolerance problems are encountered when attempting to manufacture a flat cross-talk inhibited cable in accordance with this patent.
By way of further background, U.S. Pat. No. 3,391,246 illustrates a general laminating process for providing ground plane flat multiconductor cables. U.S. Pat. No. 3,168,617, as well as French Pat. No. 1,175,923, also illustrate laminating processes for providing multiconductor flat power cable for wallpaper.
With respect to termination and stripping of prior art cables, reference is made to U.S. Pat. No. 3,547,718 which illustrates stripping of a central portion of the cable to expose the conductors. However, nowhere in this patent is a ground plane or other cross-talk inhibition utilized. Additionally, U.S. Pat. No. 3,522,652 illustrates the stripping away of the top layer of a flat cable to provide an end termination therefor.
It will be noted that U.S. Pat. Nos. 3,413,405; 4,283,593 and 4,297,522 illustrate flat cables with exterior shielding as opposed to interior shielding. U.S. Pat. No. 4,155,613 illustrates the encapsulation of parallel wires surrounded by a plastic jacket for use as a very thin flat telephone cable. This patent graphically illustrates the pitch of the cable as the distance from the center of one conductor pair to the center of another conductor pair. Typically, the term "pitch" refers to the spacing between conductor pairs and in general for many typical telephone systems the pitch is on the order of 0.0425 inches.
Finally, U.S. Pat. Nos. 3,079,458; 3,090,825; 4,149,026; 4,154,977; 4,165,559; 4,258,974; and 4,287,385, as well as British Pat. No. 1,198,739 illustrate various electrical flat cable configurations.